The present invention generally relates to implantable prostheses and to methods for making and treating same in order to substantially prevent cracking or crazing thereof when they are implanted, the treatment including applying a silicone rubber material to an implantable polymeric surface of a medical prosthesis, the polymeric surface being one that will crack when subjected to implantation for substantial time periods if it is not thus treated with the silicone rubber material. The implantable polymeric surface is made of a material that has a surface tension which is greater than that of the silicone rubber, the silicone rubber being a substantially non-polar material which is conveniently applied by immersing the medical prosthesis to be treated into a composition containing the silicone rubber.
Several biocompatible materials which are quite suitable for use in making implantable medical devices that may be broadly characterized as implantable prostheses exhibit properties that are sought after in such devices, including one or more of exceptional biocompatibility, extrudability, moldability, good fiber forming properties, tensile strength, elasticity, durability and the like. However, some of these otherwise highly desirable materials exhibit a serious deficiency when implanted within the human body or the like, such deficiency being the development of strength reducing and unsightly cracks which, for prostheses components having relatively thin strands or members, cause a complete severance of a number of those strands or members. Often, surface fissuring or cracking occurs after substantial exposure, which may be on the order of one month or more or shorter time periods depending upon the materials and the conditions, to body fluids such as are encountered during in vivo implantation and use. Many implantable prostheses are intended to be permanent in nature and should not develop any substantial cracking during years of implantation.
Several theories have been promulgated in attempting to define the cause of this cracking phenomenon. Proposed mechanisms include oxidative degradation, hydrolytic instability, enzymatic destruction, thermal and mechanical failure, immunochemical mechanisms and imbibition of lipids. Prior attempts to control surface fissuring or cracking upon implantation include incorporating antioxidants within the biocompatible polymer and subjecting the biocompatible polymer to various different annealing conditions, typically including attempting to remove stresses within the polymer by application of various heating and cooling conditions. Attempts such as these have been largely unsuccessful.
A particular need in this regard is evident when attempting to form prostheses with procedures including the extrusion or spinning of polymeric fibers, such as are involved in winding fiber-forming polymers into porous vascular grafts, for example as described in U.S. Pat. No. 4,475,972, the subject matter thereof being incorporated by reference hereinto. Such vascular grafts include a plurality of strands that are of a somewhat fine diameter size such that, when cracking develops after implantation, this cracking often manifests itself in the form of complete severance of various strands of the vascular graft. Such strand severance cannot be tolerated to any substantial degree and still hope to provide a vascular graft that can be successfully implanted on a generally permanent basis whereby the vascular graft remains viable for a number of years.
Numerous vascular graft structures that are made from spun fibers appear to perform very satisfactorily insofar as their viability when subjected to physical stress conditions that approximate those experienced during and after implantation, including stresses imparted by sutures and the like. For example, certain polyurethane fibers, when subjected to constant stress under in vitro conditions, such as in saline solution at body temperatures, do not demonstrate cracking that is evident when substantially the same polyurethane spun vascular graft is subjected to in vivo conditions. Accordingly, while many materials, such as polyurethanes, polypropylenes, polymethylmethacrylate and the like, may appear to provide superior medical devices or prostheses when subjected to stresses under in vitro conditions are found to be less than satisfactory when subjected to substantially the same types of stresses but under in vivo conditions.
There is accordingly a need for a treatment which will impart crack preventative properties to polymers that experience surface fissuring or cracking under implanted or in vivo conditions and which are otherwise desirable and advantageous in connection with the formation of medical devices or prostheses that must successfully thwart the cracking phenomenon even after implantation for months and years, in many cases a substantial number of years. Exemplary medical devices or prostheses for which such a treatment would be significantly advantageous include vascular grafts, intraocular lens loops or haptics, pacemaker lead insulators, permanent sutures, diaphragms for artificial hearts, prosthetic heart valves, and the like. Moreover, experience has shown that crack prevention that is successful under in vitro conditions is not necessarily successful under in vivo conditions.
Objectives of this type are met by the present invention which achieves a successful treatment of biocompatible polymers including polyurethanes, polypropylenes, polymethylmethacrylate and the like to the extent that these polymers do not exhibit the surface fissuring, cracking or crazing phenomenon which they would otherwise exhibit under in vivo conditions. The invention includes treating such polymers with a crack preventative material that includes a silicone rubber, typically a siloxane. The treatment can be carried out by a procedure as straightforward as dipping the prosthesis into a container including the silicone rubber material and a crosslinker or curing agent, preferably followed by taking steps to insure that the silicone rubber material adsorbs into and on the biocompatible surface of the prosthetic device at least to the extent that the crack preventative is secured to the biocompatible surface of the implantable device or medical prosthesis. Alternatively, the biocompatible surface can be pretreated with primer or other material or radiation that provides the surface with chemical functionality with which the silicone rubber material can react and to which it can bond.
It is accordingly a general object of the present invention to provide an improved implantable device, method of its production, and crack prevention treatment.
Another object of this invention is to provide an improved vascular graft that is made from spun fibers and that exhibits an exceptional ability to prevent the formation of cracks and strand severances upon implantation for substantial time periods such as those experienced in generally permanent implantation procedures.
Another object of the present invention is to provide an improved crack preventative treatment procedure for biocompatible polymers having a surface tension greater than that of a silicone rubber crack preventative agent.
Another object of the present invention is to provide an improved production method, treatment method and treated product that imparts in vivo crack prevention properties to biocompatible polymeric materials that exhibit desirable medical properties but experience cracking in in vivo applications.
Another object of this invention is to provide an improved treatment method, product and process for preparation thereof which dramatically improves the crack resistance properties of a biocompatible material while also imparting lubricating or friction reduction properties thereto.
These and other objects, features and advantages of this invention will be clearly understood through a consideration of the following detailed description.